Hi Tim! I’d like to suggest another route for you. You can use a plugin named Toolbox: Toolbox plugin - collection of utility elements , and just write your formula in it so it gets calculated in a much more stable and scalable way.
You would write something like (4*2)+15 and then have the result returned to you.
There’s also the Math.js plugin (local) but I can’t find the link. Just type in “Math” in the plugin search and you’re done.